In recent years, for use in vehicle suspension systems, various dampers have been developed that can variably control the damping force either in a stepwise fashion or continually. The damping force can be varied not only mechanically by changing the opening area of the orifice provided in the piston by means of a rotary valve but also magnetically by using a magneto-rheological fluid (MRF) for the working fluid, and controlling the viscosity of the fluid by means of a magnetic fluid valve. A vehicle equipped with such a variable damper allows the motion stability and the ride quality of the vehicle to be improved by variably controlling the damping force of the damper according to the operating condition of the vehicle.
As a technology for improving the ride quality, the skyhook control based on the skyhook theory is known. To control the ride quality (vibration) with the skyhook control, the sprung velocity is required to be detected in order to determine the target damping force such that the vertical motion of the sprung mass may be favorably controlled. It is also necessary to detect the stroke speed or the relative velocity between the sprung mass and the unsprung mass in order to perform the skyhook control because the damping force varies in dependence on the stroke speed even when the orifice area and the viscosity of the MRF remain constant.
In a conventional suspension control system performing the skyhook control, each wheel was required to be fitted with a vertical G sensor and/or a stroke sensor for detecting the vertical velocity and the stroke speed of the sprung mass. Such a stroke sensor is required to be placed within the wheelhouse or in an adjacent part of the vehicle body so that there may be some difficulty in finding a space for mounting the stroke sensor. To circumvent such a problem, a suspension control system was developed that computes the relative velocity between the sprung mass and the unsprung mass from variations in the wheel rotational speed and controls the damping force of the damper according the computed relatively velocity and other data without using a stroke sensor. See Patent Document 1.